Monitor your network (professionaly)

Professionaly and free with these two open source application: Zabbix and Nagios.

ZABBIX offers advanced monitoring, alerting and visualisation features today which are missing in other monitoring systems, even some of the best commercial ones.
http://www.zabbix.com/

Nagios is an open source host, service and network monitoring program.
http://www.nagios.org/